1. Field of the Invention
The present invention relates to a differential successive approximation (SAR) analog to digital converter (ADC). In particular, the present invention relates to a differential SAR ADC and a method of performing successive approximation for analog to digital conversion based on a capacitor array.
2. Discussion of the Related Art
SAR ADC converters generally comprise one or more n-bit converters that operate in parallel. Each n-bit converter is arranged to sample an input voltage, and generates an n-bit digital value corresponding to the amplitude of the input voltage.
Generating the n-bit digital value generally involves sampling each component of the differential input signal by corresponding sets of capacitors each having binary weighted capacitance values corresponding to the most significant to least significant bits of the n-bit output signal. The sets of capacitors are coupled to respective inputs of a comparator, and a trial and error process is then used to determine, for each capacitor in turn, what combination of high and low voltages coupled to each capacitor balances the comparator.
The operating speed of such a converter is determined to some extent by the speed at which the capacitors may be recharged between each bit decision, and this is determined by the amount of charge that needs to be drawn from the supply voltages after each bit decision. If the charge can not be drawn fast enough to recharge the capacitors, the signature of the input signal may remain on the capacitors, leading to a non-linearity in the operation of the converter. Furthermore, the more charge that needs to be drawn, the greater the power consumption of the converter.
One option for reducing the amount of charge that needs to be transferred from the supply voltages would be to reduce the size of each of the capacitors. However, this can increase the noise and reduce linearity leading to errors in the output signal.
Thus there is a need for an improved differential converter that draws a reduced charge from the supply voltages.